This text is to be filled in your mail client config’. You will set there also your various choices for your GPG signatures.

Sorry to repeat, maybe it looks scary, but you should definitely avoid all webmails (gmail, yahoo, and more generally, all mailboxes which you access with your web browser - Firefox, Chrome, Internet Explorer…).
You would have to trust the website and web connection safety, and gpg cryptography is not really OK this way. This is why I support only desktop mail clients.

Kmail

With Kmail, it’s Settings > Configure Kmail… > Identities.

There, you will find cryptography options, where you set which private key to use to sign your mails.

You will also indicate which public key is to use when encrypting mail to yourself (pretty good way to share information across devices, like a wifi password !)
About format, it is better to use OpenPGP/Mime rather than inline.

It is also important to set your preference about mail composing in Settings > Configure Kmail… > Composing.

I checked almost everything except Always show the encryption keys for approval.
The options are pretty well described for you to understand if you want to use them or not.

Thunderbird

In Tools > Account Settings, select OpenPGP Security under the address you want.
Select the option Enable OpenPGP support (Enigmail) for this identity and then Use email address of this identity to identify OpenPGP key.

You can then choose your defaults settings : encrypt, sign or not, and if you want to use PGP/Mime, which I recommend.

You have also some good options to set in Enigmail > Preferences.

Evolution

Edit > Preferences

Select Mail accounts, then the needed account and click Edit.
In the following account editor, go to the Security tab on the far right, and in the field PGP/GPG Key ID : copy the 8 characters ID that your keys manager gave you.
Remember to set your default options.

When writing a new mail, in the Security menu, click on PGP Sign and/or PGP encrypt.

Shall you sign and encrypt all your mail ?

This question is more of ethical nature. It is a personal choice.

Anyway, your software has certainly some big buttons that just want to be used to realise the cryptographic operations.

I like Phil Zimmerman’s thoughts on the matter:

What if everyone believed that law-abiding citizens should use postcards for their mail?
If a nonconformist tried to assert his privacy by using an envelope for his mail, it would draw suspicion.
Perhaps the authorities would open his mail to see what he’s hiding.
Fortunately, we don’t live in that kind of world, because everyone protects most of their mail with envelopes.
So no one draws suspicion by asserting their privacy with an envelope. There’s safety in numbers.
Analogously, it would be nice if everyone routinely used encryption for all their email, innocent or not, so that no one drew suspicion by asserting their email privacy with encryption.
Think of it as a form of solidarity.

Remember, you can always sign all your mail. It’s pretty rare that it will cause trouble to your pen pals (mostly because they have a bad/old email client).

A signed mail is authentic. Your pen pal will be sure that it comes from you and that it has not been changed on the path. But it’s still possible for everybody to read it.

But you can encrypt your mails only if your pen pals use GPG also, because you need their public key to encrypt mail for them.

Exercise

Today, I am going to ask you to simply send me a signed mail. Bare simple !

This is why I asked you to send me your key in the last article: I need it to check your signature. Checking your signature, I can insure you that you understood this part of the tutorial.

So, let’s do it : write a mail to Tuto-gpg @ 22decembre.eu. You can say what ever you would like to say, send a picture, make a comment about the tutorial, say that you love me…

Before sending, use the Sign option, or the button.
If, like me, you setted your mail software to sign all your mail, you have nothing more to do than hitting the send button now.